We had considered taking an interest rate fixed period of 15 or 20 years back then. We decided on 15 years and consciously chose the rate a bit lower, since parental leave was also upcoming and it was unclear how many hours my wife would be working afterwards. Everything else was supposed to be covered by special repayments. I believe we had already factored in about 1.5% special repayments per year for our calculations after parental leave. That we have already made 3 times the maximum amount of 5% in special repayments just 1.5 years after moving in, we never would have expected before. This was partly because there was more capital left at the end of the construction than expected, but also due to increased income. We will not be able to maintain 5% annually over the long term because at some point existing reserves will need to be increased, a car purchase will be due, etc. Nevertheless, the special repayments already made have reduced the outstanding debt after 15 years to such an extent that a 20-year loan would never pay off. So if you are really able to put aside a certain amount X every month, then you can definitely plan that into the calculation. But it must not remain just a "want". That requires some discipline and you must not lie to yourself about your monthly expenses beforehand, but should rather plan enough buffer for new purchases (car, household appliances, furniture, etc.), vacations, and repairs.